1. Field of the Invention
This invention relates to the art of thermal transfer printing and more particularly, to a thermal transfer sheet using thermally sublimable dyes and adapted for use in full color hard copies obtained by printing apparatus of the thermally sublimable dye transfer type.
2. Description of the Prior Art
In recent years, a variety of color image or picture reproducing apparatus such as home video tape recorders, computer graphic systems and the like have been widely spread, so that apparatus for outputting reproduction image information have been expected. As such an outputting apparatus, there have now been developed printing apparatus including ink jet systems, electrophotographic systems and thermal transfer systems. Among these printers, the printer of the thermal transfer type is advantageous in that it has a simple mechanism with ease in handling, that noises during the printing are not so high, and that the resultant full color images have good gradation properties.
The printers of the thermal transfer type can be broadly divided into two classes. One class makes use of thermally sublimable dyes as a colorant and the other class makes use of a hot melt comprised of a thermally fusible binder and a pigment as a colorant. The fundamental printing procedure using these printer systems is as follows. A thermal transfer sheet having a substrate provided with an ink layer on one side thereof and a sheet to be transferred are superposed so that the ink layer is facing to the sheet to be transferred. The superposed sheets are heated in accordance with information signals with a thermal head of the printer, for example, from the side of the thermal transfer sheet. As a result, the colorant in the ink layer of the thermal transfer sheet is transferred to the sheet to be transferred in an imagewise pattern.
When comparing the thermal transfer systems of the sublimation type and the hot melt type, it is generally accepted that the the sublimation-type system is poorer in dyeing density and storage stability of the thermal transfer sheet than the hot melt-type system. However, the sublimation-type system exhibits higher resolving power than the hot melt-type system with printed images of higher quality being obtained. Recent developments of sublimation-type printer tend to make the best use of the above advantage. In particular, since the density of picture elements of the thermal head increases, a further tendency toward high-quality printing is shown.
However, for obtaining clear printed images with high density by the sublimation-type thermal transfer system, it is necessary that a sublimable dye be contained in the ink layer of the thermal transfer sheet at a relatively high ratio to binder. When such a thermal transfer sheet is used for printing, a soiling-on-background phenomenon where an additional dye undergoes color development in portions other than those where images are to be formed will occur with a passage of storage time for the sheet. This will cause a lowering in quality of the printed image.